A rolling mill, which is hereinafter be referred to by the symbol PPM, is a machine which, because of its known high productive capacity, is advantageously used in pipe factories for manufacturing axially bored cylindrical blooms. A PPM comprises a rolling housing of the aforementioned kind together with an inlet frame extending parallel to the rolling axis and bearing a hydraulic or mechanical pusher rod for introducing a square bar between the cogging rollers. A second frame, likewise parallel to the rolling axis but at the outlet side of the housing, bears a rod which is hydraulically or mechanically driven in the direction of the rolling axis and bears a bit on which the square bar is rolled.
The second frame bears the apparatus and devices for withdrawing the cylindrical bloom from the bit after it has been rolled and for cooling the bit before it is used for manufacturing another cylindrical bloom.
In the case of a PPM, as in all complex machines of the same kind, special care is needed in the tooling-up operations which have to be performed before each production campaign, e.g. whenever, for manufacturing reasons, it is necessary to change the dimensions (i.e., change the calibre) of the perforated blooms to be manufactured, i.e., of the square bars for treatment.
The operation of tooling-up and sintering the rolling housing must be performed as quickly as possible so as not to affect the productivity of the machine.
In the prior art, in order to reduce the unproductive period, it is necessary to have a spare housing ready to replace the housing which has been previously in operation.